There are many procedures for the production of acrylonitrile. This compound has become one of the most important and promising organic chemical intermediates available. It is a particularly desirable intermediate in the manufacture of a wide range of products, for example, plastics, synthetic rubber, synthetic fibers, soil conditioners and the like. For many uses, acrylonitrile must be of high purity and, for this reason, strict specifications must be met in the commercial manufacture of acrylonitrile.
"Each of the commercial procedures used for the preparation of acrylonitrile produces its own set of impurities and by-products and each presents its own problems of purification." (See U.S. Pat. No. 3,459,639. )
Different purification processes are required in that different processes for making acrylonitrile result in the formation of different by-product contaminants. Accordingly, different procedures may be required to remove the contaminants and purify the acrylonitrile. Due to such constraints, any given acrylonitrile purification process is not likely to be universally interchangeable with respect to its usefulness in the purification of all acrylonitrile containing compositions.
As indicated in U.S. Pat. No. 4,404,064, one very good and commercially practiced method of producing olefinically unsaturated nitriles is the catalytic reaction of ammonia and an olefin. For example, acrylonitrile and methacrylonitrile may be produced by the vapor phase catalytic oxidation of propylene and isobutylene, respectively, in the presence of ammonia. In these processes, significant amounts of impurities are produced. The production of acrylonitrile from ammonia and propylene results in the formation of significant quantities of acetonitrile, propionitrile, acetone and the like. It is necessary to remove these by-product impurities to produce an unsaturated nitrile suitable for polymerization to other products.
U.S. Pat. No. 3,459,639 to Borrel et al discloses a process for the purification of a complex mixture of acrylonitrile, acetonitrile and other materials formed in the vapor phase conversion of acrolein or propylene to acrylonitrile over a catalyst in the presence of ammonia and oxygen. Separation of acrylonitrile from acetonitrile is accomplished by extractive distillation using deionized water at a pH of at least 5 and preferably 5-7 with the introduction of an alkaline agent to the distillation mixture.
U.S. Pat. No. 4,377,444 to Wu relates to the recovery and purification of olefinic nitriles and more particularly pertains to an improved process for the recovery and purification of olefinic nitriles, such as methacrylonitrile and acrylonitrile, produced by the ammoxidation of isobutylene and propylene from mixtures of said olefinic nitriles with such materials as acetonitrile, hydrogen cyanide, propionitrile, butyronitrile, methacrolein, acrolein, acetone, acetaldehyde, etc.
Wu points out that when an olefin, such as isobutylene or propylene, is allowed to react with ammonia and molecular oxygen in the vapor phase at elevated temperatures and in the presence of an ammoxidation catalyst, the corresponding olefinic nitriles, such as methacrylonitrile and acrylonitrile, are produced along with varying amounts of by-products of the ammoxidation reaction including acetonitrile, hydrogen cyanide, propionitrile, butyronitrile, methacrolein, acrolein, acetone, acetaldehyde, and mixtures of the desired olefinic nitrile, and some of these by-products appear in the ammoxidation reactor effluent.
In accordance with the Wu process, the products of the ammoxidation reaction are recovered in a first step by absorption in water during which step some heavy or high-boiling organic compounds are formed through polymerization, condensation, etc., of some of the lighter organic products. Accordingly, the Wu process is an improved method for separating the olefinic nitriles from the by-products formed in the ammoxidation reaction as well as from the heavy organic compounds.
The process disclosed in U.S. Pat. No. 3,051,630 to Hadley et al also relates to the purification of acrylonitrile. However, this process is particularly applicable to the purification of acrylonitrile produced by the catalytic vapor phase reaction of acrolein with ammonia and molecular oxygen. In such reactions, the crude acrylonitrile is usually recovered in the form of a dilute aqueous solution, which also contains varying amounts of acrolein and hydrogen cyanide, by contacting the gaseous reaction product with water, preferably after neutralization of any unreacted ammonia.
Once a purified form of acrylonitrile is obtained, the acrylonitrile monomer is used to produce a variety of products as indicated above. The present invention relates to a process for purifying acrylonitrile from contaminants when excess amounts of acrylonitrile are reacted with another reactant to produce a product. More specifically, the product produced using excess amounts of acrylonitrile (which is itself an intermediate) is 2-acrylamido-2-methyl propane sulfonic acid (sold under the trademark AMPS.sup.R by The Lubrizol Corporation). During the production of AMPS.sup.R stoichiometric excesses of acrylonitrile are used. Accordingly, varying amounts of acrylonitrile remain unreacted and are present with other contaminants after the desired product is formed and separated away. These contaminants are often quite reactive with acrylonitrile causing polymerization, more specifically causing undesired copolymerization of acrylonitrile monomer units with monomer units of contaminants present. If the unreacted acrylonitrile which is present with contaminants is merely recycled in the synthesis of the 2-acrylamido-2-methyl propane sulfonic acid, the resulting product does not have the desired degree of purity and will not meet desired specifications. Further, any such contaminated acrylonitrile could initiate unwanted polymerization. In addition, any reaction product (such as 2-acrylamido-2-methyl propane sulfonic acid) produced from such contaminated acrylonitrile would not have the desired specifications, e.g., lack of water insoluble polymerized particles, degree of purity, etc. Accordingly, the present inventors have developed an improved process for the purification of such unreacted acrylonitrile.
As pointed out above, processes for purifying acrylonitrile vary based on factors such as the contaminants present with the acrylonitrile. The present invention is different from the processes discussed above in that those processes relate generally to the purification of waste from acrylonitrile production whereas the present process relates to the purification of unreacted acrylonitrile from the waste stream created during the manufacture of a product (such as 2-acrylamido-2-methyl propane sulfonic acid) produced using excess amounts of acrylonitrile. In other words, the present invention is directed to a process for purifying unreacted acrylonitrile when the acrylonitrile had been used as a reactant to produce another product and the prior art such as that discussed above is generally directed to purifying acrylonitrile away from unreacted reactants used to produce acrylonitrile. These unreacted reactants formed while producing acrylonitrile are generally less reactive than the contaminants present with the unreacted acrylonitrile from making 2-acrylamido-2-methyl propane sulfonic acid. The more reactive the contaminants the greater the need for removing such contaminants.
When different reactants are used, different contaminants result. Accordingly, no one process is necessarily interchangeably useful in connection with purifying acrylonitrile away from all types of contaminants.